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Center for the Built EnvironmentUC Berkeley

Peer Reviewed

Title:Moving Air for Comfort

Author:Arens, Edward, Center for the Built Environment, University of California, BerkeleyTurner, StephenZhang, Hui, UC BerkeleyPaliaga, Gwelen

Publication Date:05-08-2009

Series:Indoor Environmental Quality (IEQ)

Publication Info:Indoor Environmental Quality (IEQ), Center for the Built Environment, Center for EnvironmentalDesign Research, UC Berkeley

Permalink:http://escholarship.org/uc/item/6d94f90b

Additional Info:Original Citation: ASHRAE Journal, May 51 (25), 8 – 18, 2009 1

Keywords:Air speed, air movement preference, standard, draft limit, warming environment, ceiling fan

Abstract:Moving air has long been used to provide comfort in warm environments. Provision for indoor airmovement was one of the wellsprings of traditional architectural design in warm regions, affectingbuilding form, components, and equipment over millennia. However, this design option has fadedfrom practice since the advent of air-conditioning, in which the focus has been on controllingtemperature and humidity. Despite the fact that air movement can be an energy efficient alternativeto air cooling, it became viewed more as a possible source of undesirable draft, and comfortstandards came to set room airspeed limits very low, even for temperatures as warm as 26ºC(79ºF). An exception was granted if the airspeed source was under personal individual control,such as a window in a private office, or a desk fan, but only above 26 ºC (79ºF).

ASHRAE Journal, May 2009, pp 8 – 18 1

Moving Air for Comfort

Edward Arens, Stephen Turner, Hui Zhang, Gwelen Paliaga

Moving air has long been used to provide comfort in warm environments. Provision for indoor

air movement was one of the wellsprings of traditional architectural design in warm regions,

affecting building form, components, and equipment over millennia. However, this design

option has faded from practice since the advent of air-conditioning, in which the focus has been

on controlling temperature and humidity. Despite the fact that air movement can be an energy-

efficient alternative to air cooling, it became viewed more as a possible source of undesirable

draft, and comfort standards came to set room airspeed limits very low, even for temperatures as

warm as 26ºC (79ºF). An exception was granted if the airspeed source was under personal

individual control, such as a window in a private office, or a desk fan, but only above 26 ºC

(79ºF).

The need to improve buildings’ energy efficiency and CO2 footprint has opened this situation to

reexamination. Air conditioning is now one of the largest end-uses of energy in developed

countries, and much of the world’s current development is in tropical or subtropical regions. We

must look closely at all opportunities to reduce the energy consumed in providing comfort. The

use of air movement is one of these opportunities. In cool environments, drafts are certainly a

problem, but how true is this for temperatures that are neutral or warm? If drafts are not a

problem at these temperatures, might we resume using air movement as a means of cooling

occupants?

Recent studies of occupied buildings give very consistent evidence that large percentages of

occupants would prefer more air movement than what they currently have, and very small

percentages would prefer less. This is true in all conditions that occupants perceived as warm,

thermally neutral, and even slightly cool. In terms of temperature, there is small risk of draft,

and a strong preference for more air movement above 22.5ºC (72.5ºF).

Responding to these findings, new provisions in ASHRAE Standard 551 allow for elevated air

speed to broadly offset the need to cool the air in warm conditions.

Building occupants’ air movement preferences

Under ASHRAE sponsorship, field studies of occupant comfort in buildings worldwide have

been assembled in a database2. In this database, there are 11 studies (comprising 53 buildings

and 6148 sets of data) that included direct questions about the occupants’ air movement

preference. These studies, from Montreal, Honolulu, and Sydney, Kalgoorlie, and Townsville in

Australia, are used for the analyses presented below. The six Honolulu buildings are all schools,

and all the others are office buildings. Except for two naturally ventilated schools and one

mixed-mode office in Sydney, all the buildings are fully air-conditioned.

Table 1 lists occupants’ stated air movement preference by the thermal sensation they reported at

the same time. They are also arranged into two speed ranges: V<0.2 m/s (39 fpm; 0.44 mph)

ASHRAE Journal, May 2009, pp 8 – 18 2

representing conditions below the draft limit in ASHRAE Standard 55 (2004), and V>=0.2 m/s

representing potentially drafty conditions (for the latter, the mean airspeed was 0.32 m/s). It is

obvious that when people felt “neutral” or warm (“slightly warm, “warm”, “hot”), a very small

percentage of them (7% or less) wanted less air speed. This is even true for the higher speed

range (V>=0.2 m/s), in which at least 93% of people accepted the higher air speed, or wanted

even more. For both speed ranges, it is only under “cold” sensation that more people “want less”

air speed than “want more”. Even under “cool” and “slightly cool” sensations, substantially

more people preferred more air speed than less.

Table 1. Air movement preferences by thermal sensation and for two ranges of airspeed, n=6148

Percentage of occupants preferring: (N) Thermal

sensation Air

speed

range

(m/s)

Want less No change Want more

Top (Std. Dev.) (ºC)

Cold (<-2.5)

0 – 0.2 33.33 46.85 19.82 111 22.66 (0.91)

>=0.2 50.00 42.30 7.69 26 23.5 (1.45)

Cool (-2.5 to -

1.5)

0 – 0.2 13.07 60.47 26.47 597 22.92 (1.08)

>=0.2 11.55 72.51 15.94 251 24.28 (2.0)

Slightly

cool (-1.5

to -0.5)

0 – 0.2 10.75 53.08 36.17 1153 23.05 (1.23)

>=0.2 11.35 62.23 26.42 458 24.59 (2.16)

Neutral (±0.5)

0 – 0.2 2.62 51.46 45.92 1407 23.30 (1.23)

>=0.2 4.62 57.26 38.12 585 24.86 (2.03)

Slightly

warm (0.5

to 1.5)

0 – 0.2 2.31 27.73 69.95 822 23.65 (1.41)

>=0.2 3.36 30.87 65.77 298 25.46 (1.85)

Warm (1.5 to 2.5)

0 – 0.2 4.24 18.37 77.39 283 23.75 (1.58)

>=0.2 4.96 28.93 66.12 121 25.79 (2.08)

Hot (>2.5)

0 – 0.2 4.55

0

95.45

22

24.96 (1.28)

>=0.2 7.14 14.29 78.57 14

26.23 (2.04)

ASHRAE Journal, May 2009, pp 8 – 18 3

The associated operative temperatures are also shownA. Under the same thermal sensation

category, the operative temperature for the higher airspeed is between 1.5 – 2 K (3 – 4ºF) higher

than the temperature for lower speed. This difference illustrates the tradeoff between air speed

and temperature in producing equivalent levels of comfort.

Air movement preference for people feeling neutral to slightly warm

The following figures summarize air movement preferences in the thermal sensation range of –

0.7 to 1.5. A sensation of -0.7 is a little cooler than the usual definition of ‘neutral’, but it

matches the cool end of the Category III range (for acceptable existing buildings) in the

European standard CEN 152513. The figures exclude sensations above 1.5 because, although

Table 1 shows that air movement is clearly welcome when sensation is above 1.5, this sensation

is too warm for normal office environments and ideally would not happen often. The –0.7 to 1.5

range should give a conservative estimate of air movement preference for neutral and slightly

warm people.

Figure 1: Under the sensation range (-0.7 to 1.5), far more people (52%) wanted more air

movement than less air movement (3%). The percentage wanting more was greater than the

percentage preferring “no change”.

Want Less: 3%

Want More: 52%

No Change: 45%

Figure 1. Air movement preference (sensation –0.7 to 1.5), all airspeeds. (n = 3230)

A The operative temperature is a measure combining the air temperature, affecting the body by

convection, and the surface temperatures of the surroundings, affecting the body by radiant

exchange. It is defined as the uniform temperature of an imaginary black enclosure in which an

occupant would exchange the same amount of heat by radiation plus convection as in the actual

nonuniform environment.

ASHRAE Journal, May 2009, pp 8 – 18 4

Figure 2: Even when their airspeed was at the higher range (>=0.2 m/s, Figure 2), a much

higher percentage of people (47%) wanted more, or accepted the higher airspeed (no change =

49%), than wanted less (4%).

Want Less: 4%

Want More: 47%

No Change: 49%

Figure 2. Air movement preference (sensation –0.7 to 1.5), airspeed >=0.2 m/s (39 fpm)

(n = 924)

Figure 3: In the surveys, occupants were also asked whether the air movement was ‘acceptable’.

29% of all occupants said it was not. Looking at these respondent’s preference votes, one can

see that the unacceptable air movement is due to too little air movement, not too much. 84%

wanted more, 7% wanted less (Figure 3).

Want Less: 7%No Change: 9%

Want More: 84%

Acceptable Air Movement

71%

Unacceptable Air Movement

29%

Figure 3. Air movement preference for people (sensation –0.7 to 1.5) who said the

airspeed was unacceptable. (n = 2091)

ASHRAE Journal, May 2009, pp 8 – 18 5

Figure 4: When people felt the airspeed was not acceptable, and when they were experiencing

the higher range (>=0.2 m/s), did they feel the speed was too much and therefore not acceptable?

Figure 4 shows that for the higher speed range, when people said that they felt the speed was not

acceptable, the unacceptability was still due to insufficient air movement. The percentage

wanting more (73%) is still far more than the percentage wanting less (17%). The difference is

now smaller but it is still very clear that the great majority found the speed unacceptable because

there wasn’t enough, even with the speed in the higher range.

Acceptable Air Movement

78%

Unacceptable Air Movement

22%

Want More: 73%

Want Less: 17%No Change: 10%

Figure 4. Air movement preference for people (sensation –0.7 to 1.5) who said the

airspeed was unacceptable, V>=0.2 m/s. (n = 324)

We also calculated the draft risk percentage (DR) for all the database’s occupants based on the

equations provided in the previous ASHRAE Standard, using airspeed, temperature, and

turbulence intensity as inputs. Then we look at the air movement preference for those people

when their DR >20% (thermal sensation again within –0.7 and 1.5, n=172), the percentage

wanting less air speed was 8%, 59% wanted no change, and 33% wanted more. The result is that

92% of the population that were predicted to feel a draft actually accepted their airspeed or

wanted it higher.

Summarizing these ASHRAE field study results, it is clear that for sensations -0.7 to 1.5, air

movement should be encouraged in these air conditioned buildings. The air movement should

not be made so great that it leaves people feeling cold, but a certain amount of it does answer a

basic need found in the surveys, and can offset an increase in temperature in the space. Similar

results have been found for a building in which occupants have personal or group control over

window ventilation4.

ASHRAE Journal, May 2009, pp 8 – 18 6

Provisions for elevated air speed in Standard 55

ASHRAE Standard 55 now provides a two-step procedure for setting a comfort zone for

temperature, radiation, humidity, and air movement. Both steps of this process can be carried

out using the ASHRAE Comfort Tool5 (available from ASHRAE publications), or interpolated

from figures in the standard. The first step, unchanged from the previous standard, combines air

temperature and radiant temperature in the index ‘operative temperature’ as defined above,

which is then combined with humidity to produce comfort zones for still air conditions displayed

on a psychrometric chart. This step uses the PMV human heat balance model, combining these

environmental variables with the occupant’s clothing level (expressed in the insulation unit ‘clo’)

and activity level (expressed in the metabolic rate unit ‘met’)6,7

.

The second step evaluates elevated air speeds. It uses a different model that better accounts for

convective cooling of the body. The Standard Effective Temperature (SET) thermophysiological

model is based on longstanding ASHRAE research and practice6,8

. Equal heat balance and skin-

wettedness for different airspeeds can be plotted in terms of SETB contours. If one starts with

the underlying PMV comfort zone, these SET contours form the boundaries of an air-movement

comfort zone in which air speed is plotted against operative temperature.

Figure 5. Elevated air speed for warm air temperatures

B

temperature, standard effective (SET): the temperature of an imaginary environment at 50% RH, <0.1 m/s air

speed, and with mean radiant temperature equal to air temperature, in which the total heat loss from the skin of an

imaginary occupant with an activity level of 1.0 met and a clothing level of 0.6 clo is the same as that from a person

in the actual environment, with actual clothing and activity level.

ASHRAE Journal, May 2009, pp 8 – 18 7

These steps are shown in Figure 5, taken from the new standard. Figure 5 shows two example

comfort zones, for the clothing insulation levels: 0.5 clo (e.g., summer short-sleeved shirt and

slacks) and 1.0 clo (winter heavy business suit), and the sedentary activity level of 1.1 met.

These clo and met levels have been traditionally used in the Standard’s first figure, which plots

PMV-based still-air comfort zones on a psychrometric chart. In Figure 5, those zones are

transposed to the bottom of the y-axis, in the region defined as ‘still air’ (0.1 to 0.15 m/s), for

50% relative humidity. The comfort zones each extend from -0.5 PMV (the thermal sensation

midway between ‘neutral’ and ‘slightly cool’) and +0.5 (between ‘neutral’ and ‘slightly warm’).

One can see some temperature dependence on airspeed within this still-air region, as predicted

by the PMV model.

Comfort envelope: Starting with these still-air zones, 0.5 and 1.0 clo comfort envelopes for air

speeds above 0.15 m/s are generated using the SET model. SET contours of equal heat-loss

relate a range of air speeds and temperatures to the equivalent PMV value in the still-air zones.

These two comfort envelopes can be interpolated to roughly account for other clothing levels,

but in practice, envelopes for any clothing, activity, and humidity level would be more accurately

determined using the ASHRAE Thermal Comfort Tool.

Limits to air speed: Figure 5 provides additional limits not based on SET. These are divided

into two categories, with and without local control.

(1) With local control. The full equal heat-loss envelope for a given clothing level in Figure 5

applies when control of local air speed is provided to occupants. For control over their local air

speed, control directly accessible to occupants must be provided for every 6 occupants (or less)

or for every 84 square meters (900 square feet) (or less). The range of control shall encompass

air speeds suitable to maintain comfort for sedentary occupants. The air speed should be

adjustable continuously or in maximum steps of 0.25 m/s (50 fpm) as measured at the occupant’s

location.

(2) Without Local Control. Within the equal heat-loss envelope, if occupants do not have control

over the local air speed in their space, limits apply as shown by the light gray area in Figure 5.

They apply for light, primarily sedentary office activities. For other types of activities or

occupancies, such as retail or warehousing, these limits may differ or not be necessary. There is

little quantitative information available for other types of occupancies.

For operative temperatures above 25.5 ºC (77.9 ºF), the upper limit to air speed is 0.8 m/s (160

fpm).

Below 22.5 ºC (72.5 ºF), the limit is 0.15 m/s (30 ft/min) in order to avoid cold discomfort due to

draft. 0.15 m/s is the upper limit of the ‘still air’ zone, and is coincidentally equal to the airspeed

self-generated by an office worker at 1.2 met.

Between 22.5 and 25.5 ºC (72.5 ºF and 77.9 ºF), the allowable speed follows an equal-SET curve

dividing the light and dark gray areas. This local-control boundary between 22.5 and 25.5 ºC is

ASHRAE Journal, May 2009, pp 8 – 18 8

not linked to any PMV comfort zone, but is based on temperatures that have been observed in

office field studies to cause virtually no risk of draft9.

Using a temperature-based local-control boundary provides a substantial practical advantage in

environmental control: decentralized air movement sources (like ceiling fans) can be

manufactured or retrofitted with temperature-based speed controllers for automatic control of

common spaces.

It is worth noting that the 1 clo zone in Figure 5 represents a higher level of clothing than is

typical for situations in which air-movement cooling is used. For more typical summer clothing

levels such as 0.5 and 0.6 clo, the limits to airspeed and temperature will be determined by the

cool-side SET boundary of the comfort zone, not by this occupant-control boundary.

Air speed measurement: Above 22.5 ºC (72.5 ºF), the overall heat balance of the body

determines comfort. For this, the standard uses the average air speed of the three measurement

heights that represent the seated occupied zone -- 0.1, 0.6, and 1.1 m (4, 24, and 43 inches).

Measurements should be taken in the occupants’ estimated vicinity. To prevent anyone from

attempting to use the standard to justify cold-air dumping from poorly designed HVAC supply

air outlets, the coldest temperature in the occupied zone must be used to represent the entire

zone. This also renders it improbable that cold air distribution systems can be used as the source

of elevated air movement because they are likely to produce cold air jets.

Below 22.5 ºC (72.5 ºF), the problem is avoiding thermal discomfort on a local, usually

unclothed, part of the body. The SET and PMV models do not distinguish between clothed and

unclothed portions of the body, so the following conservative approach is adopted: the maximum

mean air speed of the three measurement heights, and the lowest air temperature is used for the

SET calculations, thereby over-predicting the whole-body cooling to a level that more closely

approximates the cooling of the most affected local part.

There is no longer a requirement to measure turbulence intensity for determining draft risk.

Validation: In lab studies where airspeed has been imposed on people, conditions on the cool

side of the ‘without local control’ (light gray zone) boundary have been found to be comfortable,

even recommended as optima10,11,12

. Imposed air movement does not appear to be a problem at

these temperatures. This is also supported in the ASHRAE field database described above: for

airspeeds above 0.4 m/s and air temperatures between 22.5 and 25.5 C, 32% wanted more air

movement, 59% no change, and only 9% wanted less.

On the warm side of 25.5 ºC (77.9 ºF), ceiling fan studies (with no occupant control) have found

1 m/s (197 fpm, 2.2 mph) acceptable to 77% of subjects13

and above (95%14

, 80-100%15

). A

frontal desktop breeze of 0.8 m/s air speed produced an acceptability level of 80%10

. Subjects in

preferred airspeed studies16,17,18

often chose airspeeds well above 1 m/s. Fountain19

summarized

many of these studies, from which one can see that the SET-based curves in Std 55 represent the

laboratory results well.

ASHRAE Journal, May 2009, pp 8 – 18 9

Air speed requirements for industrial occupancies: Standard 55 has also added an informative

appendix to address more strenuous indoor activities than office work, such as industrial work.

For such work in hot environments, sweating may be acceptable to the occupants. Elevated

airspeeds provide high rates of cooling by sweat evaporation. Using SET as described above,

hot conditions with high airspeeds may be equated to their temperature equivalents at lower

airspeeds. There is no specified limit to air speed in this range.

Impact in practice

The new provisions allow designers to use fans, stack, or window ventilation to offset

mechanical cooling, or in some climates to supplant it entirely. The following two examples are

of offices using ceiling fans.

City Hall, Orinda CA (Figure 6). Ceiling fans are used to enable a mixed-mode system that

combines passive cooling through operable windows and a stack vent with indirect-direct

evaporative cooling, instead of compressor-based cooling. The building is 1300 m2 (14,000

sq.ft.) with 40 full time occupants along with a community meeting room and conference rooms.

The building management system controls clerestory window opening based on indoor-outdoor

temperature differential and illuminates signs that instruct occupants to open/close their

windows. A total of thirty six fans with 52” blade diameter are provided in office, conference

and circulation spaces. Figure 6b shows two spaces, one with four fans arrayed in a 100 m2

(1,076 sq. ft.) open space (left image), another with 10 fans arrayed for 273 m2 (2,938 sq. ft.) of

cubicles and hallways (right image). Fans are controlled individually or in groups of two or

three from wall mounted switches and have three speed levels. The fans use approximately 30-70

W/fan. In the design, they were assumed to allow the air temperature to increase 2.6K (4.7F) at

0.75 m/s. Simulations of indoor temperature and comfort predict that the fans are critical to

maintain comfort for 100-200 hours per year when the evaporative cooler has limited capacity

and space temperatures rise.

The building has been occupied for a year and a half and the users are happy with the fans.

Figure 6a Orinda City Hall plan view. Orange circles are the ceiling fans, and the light orange

circles are the areas in which 0.75 m/s (150 ft/min) air velocity is supplied in the occupied zone).

ASHRAE Journal, May 2009, pp 8 – 18 10

Open space Cubicles and hallway

Figure 6b. Orinda City Hall with ceiling fans

Loisos+Ubbelohde Office, Alameda CA (Figure 7). This 2,400 square foot design office is

cooled only by a system of operable windows, automated shading, and ceiling and desk fans.

The occupancy is 18, for an occupant density of 133 sq ft/ person. Ceiling fans are individually

controllable, from one control panel in the entry area. They are controlled in groups of one or

two. Fans have high-efficiency twisted airfoil blades designed by Florida Solar Energy

Center/AeroVironment, generating approximately 40% higher CFM/W than paddle-blade fans,

drawing between 9 to 50W from low to full speed (0.4 to 1.6 m/s)20

.

System has been in place through one summer in its present configuration, with owner and

occupants expressing satisfaction. Owner feels that the fan density shown could be increased for

more even coverage. He suggests that air motion sectional diagrams, similar to photometric

curves, be published to facilitate the design layout of fans in accordance with work stations.

Ceiling fans in open office space Ceiling fan

Figure 7 Design office in Alameda CA

ASHRAE Journal, May 2009, pp 8 – 18 11

Conclusions

Standard 55-2009 includes new provisions for using air movement to offset warm air

temperatures. They are based on field studies that consistently demonstrate that in neutral and

warm environments, people want more air speed. The SET model is used to generate comfort

zones in which elevated air speed offsets warm air temperature. New criteria for group local

control are specified. The new air movement provisions assist the design of both passive and

mechanical systems such as natural ventilation, thermal mass cooling, evaporative cooling, and

mixed-mode HVAC. Two examples are given of applications in open-plan offices.

Acknowledgements

The authors wish to acknowledge Tyler Hoyt and Elliot Nahman for their assistance with the

statistical analysis and graphics.

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